Obstetricians perform more cesarean sections when there are financial incentives to do so, according to a new study that explores links between economic incentives and medical decision-making during childbirth.

About 1 in 3 babies born today is delivered via C-section, compared to 1 in 5 babies delivered via the surgical procedure in 1996. During the same time period, the annual medical costs of childbirth in the U.S. have grown by $3 billion annually. There are significant variations in the rate of cesarean deliveries in different parts of the country — in Louisiana, for example, the C-section rate is nearly twice as high as in Alaska.